I noticed a certain plant growing on my lawn this year that I haven't
seen before. I took a couple of pictures. Can someone tell me if I
have something to worry about. Right now they are growing in a spot
where I find it tough to grow grass and they aren't really noticeable
unless you look up close. I'm worried they might take over the entire
lawn though. Here are the pics:

http://img206.imageshack.us/img206/5151/weedsdq4.jpg

http://img78.imageshack.us/img78/9940/weeds2ws6.jpg